This invention relates in general to a system for displaying information on liquid crystal display (LCD) devices, and in particular, to low power LCD with gray shade driving scheme.
Liquid crystal displays are used in a variety of devices such as cell phones, pagers, and personal digital assistant devices. Since many of the uses of these displays are in portable, battery operated devices, low power consumption is an important display attribute. Many prior art systems, such as LCD displays, include circuitry to provide power to the display through row and column electrodes whose overlapping regions form pixels. Information to be displayed is converted into row addressing and column data signals according to one of a variety of techniques. These techniques work within the physical limitations and specifications of the LCD material by providing the appropriate signals to the display electrodes.
Typical for use in passive LCD displays are multiplexing techniques that are based on the principle that the optical properties of the display respond to root mean square (R.M.S.) signals applied to each individual pixel. Common implementations of this technique, such as the Alto-Pleshko Technique, use row signals to select rows for receiving information and the column signals as data signals to carry information to be presented. Variations of this technique have been developed to drive displays using alternating current (AC) to limit direct current (DC) damage to liquid crystals, and to keep the applied voltages within certain ranges. This variation of display technology is exemplified by the Improved Alt and Pleshko Technique (IAPT). In addition to the IAPT approach to controlling displays, there are many other schemes that can be applied in conjunction with the basic IAPT techniques for generating gray shades in the displays, such as frame rate modulation (FRM) and pulse width modulation (PWM) for producing multiple gray levels. Specifically, prior art techniques limit scanning to certain set patterns by scanning rows consecutively from one edge of the display to the opposite edge.
It has been a continuing goal of LCD display development to reduce power requirements, allowing, for example, for prolonged battery lifetime in portable devices. Among the approaches that have been attempted to reduce the power requirement are: development of new crystals, the incorporation of more advanced electronics into the display, and developing computationally intensive display driver algorithms, such as MLA techniques. The present invention introduces a new, low-power LCD panel addressing scheme that uses simple driving algorithms and that is compatible with existing liquid crystal materials and LCD manufacturing technology.
Referring to FIG. 1, a typical configuration of passive LCD and its driving waveform is illustrated. As demonstrated in the LCD panel 10 of FIG. 1, panel 10 includes an array 12 of N elongated row electrodes and an array 14 of M elongated column electrodes, where N, M are positive integers. The two arrays of electrodes are arranged transverse to one another so that each row electrode intersects and overlaps each column electrode at an overlapping area, where the overlapping area when viewed in a viewing direction by a viewer (such as the direction 16 perpendicular and into the plane of the paper in FIG. 1) defines a pixel, such as pixels 18 as shown in FIG. 1. The row and column electrodes are driven by circuits 22, 24 as shown. Following the convention of the industry, row and column electrodes are also referred to below as COM and SEG electrodes respectively, the selection (addressing) and data signals applied thereto referred to as below the COM and SEG signals or pulses respectively, and circuits 22, 24 are also known as row (COM) and column (SEG) drivers respectively.
When the driver 22 applies voltages or electrical potentials to the COM electrodes, a voltage is applied to each of the row electrodes for a time period referred to below as the row scanning or addressing period, or line period. The voltages or potentials are applied to the row electrodes at a frequency or rate referred to below as the line rate or the row scanning or addressing rate. When a voltage of “non-scanning” value is applied to a row electrode that is selected for addressing, no image will be displayed in the pixels overlapping such row electrode irrespective of the values of the voltages applied to the SEG electrodes, and when a voltage of “scanning” value is applied to a selected row electrode for addressing, a line of an image will be displayed in the pixels overlapping such row electrode. By applying scanning voltages to the N row electrodes sequentially while appropriate data SEG pulses are applied to the column electrodes, line images are displayed forming a full image comprising multiple lines.
To enhance the content of an information display, it is generally desirable to produce multiple gray levels in the display. Such gray shades are generally achieved by two conventional methods in STN (Super Twisted Neumetic): pulse width modulation and frame modulation.
In a pulse width modulation (PWM) scheme, within each line period, the SEG pulses are modulated such that for x % of the line period the SEG output level is at voltage V1, and for the rest of the (100−x) % of the line cycle, the SEG driver output level is at a lower voltage V0, and the resulting VRMS across the pixel electrode will have a value approaching x % of the voltage difference between the V0 and V1 above V0.
In a conventional type of frame rate modulation (FRM), multiple frames with different gradations of gray shades are grouped together as a set, where the frames are applied for the same line period, and the signals are distributed over the entire set to produce the final shading through the root mean square (RMS) averaging effect of STN. For example, a set may consist of 15 frames. Then for levels 0˜15, the data can be distributed over this set of 15 frames and achieve the gray shading effect.
Both of these conventional scheme consume significant power. In the case of Pulse Width Modulation, first consider a case where the whole screen is to display a constant 50% shading. This would result in the SEG toggling at twice the line rate (ON-OFF-ON-OFF) and consume very significant power due to the capacitor loading effect on the SEG electrodes. Due to this very high toggle rate and power consumption, PWM scheme generally experience high fluctuation of power consumption, and can cause problems in system design.
As for Frame Rate Modulation, the RMS effect of STN has a bandwidth limit. In order to minimize the visible flicker, the full set of frames needs to be repeated faster than 60 Hz, which is the threshold of human flicker detection. For example, to produce 16 shades, a set of 16 frames are required, and the full frame need to be repeated at 60×16=960 fps (frames-per-second). Although spatial dithering (such as 2×2 matrix) can be used to reduce that frequency by up to ¼, but 240 fps is still significantly higher the 60 Hz which is typical for pure black and white (B/W) STN LCD (i.e. without gray shade), and therefore would consume almost four times of the power consumed by pure black and white (B/W) STN LCDs.
Another short coming of the conventional Frame Rate Modulation scheme is the resulting shading is linearly spaced between V0 and V1, where the STN LC material always has a S shaped VRMS to transmittance curve as illustrated in FIG. 4. Linearly spaced modulations cause gray shades at the two end of the spectrum (level 1˜4, and level 13˜16) to become indistinguishable from one another. In order to accomplish such curve compensation, significantly higher than 16 frames will be required. And the power consumption can increase very significantly.
Another aspect of the present invention is related to the more modem LCD control scheme such as Scheffer's Active Addressing, or Multi-Line-Addressing, where more than one row of pixels is being addressed during each line period. For example in a typical configuration of MLS with L=4, four rows of pixels are addressed simultaneously, and each SEG signal will need to be calculated based on the desired states of the four rows of pixels. If the PWM scheme is used, then each line period can be further divided into 5 subperiods, depending on where each of the four pixels will need to transition in order to achieve the desired shades. This can increase the amount of SEG switching activity by 5 times, and practically rendered PWM impractical for any system employing the MLS driving scheme. It is therefore very desirable to find a new gray shade scheme where the SEG signal will remain constant during each line period, while achieving desirable VRMS modulation to produce the desirable gray shades.
None of the above-described LCD driving schemes are entirely satisfactory. It is therefore desirable to provide improved LCD driving schemes for producing gray shades with minimum increases of power consumption as compared to pure black and white LCDs. It is also desirable to provide a driving scheme for suppressing flicker with further reduction in power consumption.